1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, and an image forming apparatus and a process cartridge using the electrophotographic photoreceptor.
2. Description of the Related Art
Recently, in terms of saving spaces in offices and expanding business opportunities, an image forming apparatus has been required to have higher speed, smaller size, produce higher quality full-color images and easy maintenance. These relate to improvement of electrical properties and durability of an electrophotographic photoreceptor to be achieved.
In order to achieve these, trials to reduce image defects due to the electrophotographic photoreceptor when used for long periods are made, and a number of developments to make the electrophotographic photoreceptor have longer life are disclosed. In order to make the electrophotographic photoreceptor have longer life, durability thereof against various hazards it receives when producing images needs improving. The hazards are broadly classified into a mechanical hazard and a chemical hazard.
As an example of the mechanical hazard, a hazard due to a cleaner removing a toner remaining on the electrophotographic photoreceptor is known. The cleaner contacts an elastic member such as a cleaning blade to the surface of the photoreceptor to forcibly remove a toner therefrom, and has large toner removability while saving space. This is known as an effective means for downsizing an image forming apparatus, but this directly contacts an elastic member such as a cleaning blade to the surface of the photoreceptor, which is frictionized thereby. Therefore, the electrophotographic photoreceptor receives such a large mechanical stress that the surface thereof is likely to be abraded. For example, Japanese published unexamined applications Nos. JP-H05-181299-A, JP-2002-06526-A, JP-2002-82465, JP-2000-284514-A and JP-2001-194813-A disclose methods of preventing abrasion of the electrophotographic photoreceptor by forming a high-hardness protection layer thereon.
As the chemical hazards, hazards due to an oxidizing gas and an alkaline gas generated when the surface of the electrophotographic photoreceptor is charged are known. When the electrophotographic photoreceptor is exposed to oxidizing gases such as ozone and nitrogen oxides generated near a charger (KONICA Technology Report Vo. 13 (2000)), a charge transport material such as a hole transport material and an electron transport material deteriorates due to the oxidizing gases (Journal of Imaging Science 32:205-210 (1998)), resulting in deterioration of properties of the electrophotographic photoreceptor. When a short-life electrophotographic photoreceptor is used, the oxidizing gas deteriorates only the outermost surface layer in many cases to keep deteriorated components in a small amount. When a long-life electrophotographic photoreceptor is used, the oxidizing gas occasionally deteriorates even an inside of the electrophotographic photoreceptor, resulting in deterioration of image density and occurrence of background fouling, i.e., it is unable to keep producing high-quality images for long periods.
In order to solve problems of the chemical hazards, Japanese published unexamined application No. JP-2006-99028-A discloses a method of preventing deterioration of the charge transport material due to the oxidizing gas by adding an antioxidant into a charge transport layer and a surface layer. In addition, in order to prevent the oxidizing gas from penetrating the charge transport layer and the surface layer, Japanese published unexamined applications Nos. JP-H03-45962-A and JP-H07-281463-A disclose methods of reducing their permeability to gas. Further, Japanese published unexamined applications Nos. JP-H09-26685-A and JP-2002-229241-A disclose methods of preventing a discharge product (oxidizing gas) from generating in the charging process.
However, even these methods do not substantially improve the deterioration because the electrophotographic photoreceptor includes oxidized and deteriorated components in a large amount, and it is still unable to keep producing high-quality images for long periods.
As another example of the chemical hazards, an electrostatic hazard due to an electrostatic stress to the electrophotographic photoreceptor is known. The electrostatic hazard is a hazard due to an electrostatic stress to the electrophotographic photoreceptor because a charge passes each layer, e.g., a surface layer, a charge generation layer, a charge transport layer and an intermediate layer when the electrophotographic photoreceptor is irradiated to remove a charge on the surface thereof in an ordinary image forming process. At present, most of the electrophotographic photoreceptors widely used are formed of organic materials. When the electrophotographic photoreceptors formed of an organic material is repeatedly charged and discharged, the organic material gradually deteriorates, resulting in generation of charge trap in the layer and deterioration of electrical properties, i.e. chargeability and optical attenuation of the electrophotographic photoreceptor.
In order to solve problems of the chemical hazards, Japanese Patent No. JP-2795566-B1 (JP-H05-142846-A) discloses a method of dispersing an inorganic particulate material to vary film resistance by an electric field intensity. In a high electric field of 1×105 V/cm, a resistivity is maintained at not less than 1014 Ω·cm. In a high electric field of 2×105 V/cm, the surface layer has lower resistivity such that the electrophotographic photoreceptor has better charge transportability and produces clear images.
However, the resistivity not less than 1014 Ω·cm in a high electric field of 1×105 V/cm does not obtain sufficient charge transportability, resulting in higher residual potential of the electrophotographic photoreceptor. In addition, the content of the inorganic particulate material of 10 to 40% by weight disclosed in the method needs a high resistivity inorganic particulate material to have the resistivity. The inorganic particulate material is short of bulk conductivity and limited current conductivity becomes dominant, resulting in noticeable increase of residual potential of the electrophotographic photoreceptor when producing images for long periods.
Further, the electrophotographic photoreceptor is required to produce high-quality images and image quality stability for long periods is essential. In order to achieve the image quality stability, improvements of toner transferability and cleanability are required. As methods of improving image quality stability, Japanese published unexamined applications Nos. JP-2009-288402-A and JP-2008-090214-A disclose methods of including a particulate fluorine material in the surface layer of a photoreceptor. However, simply including a particulate fluorine material in the surface layer is not sufficient for image quality stability including durability of the photoreceptor when used for long periods.
As mentioned above, in order to achieve longer life of the electrophotographic photoreceptor, it needs to have durability against the mechanical and chemical hazards. The present inventors particularly pay attention to solve problems of the chemical hazard, and try to radically solve them by reducing the charge transport material included in the surface layer to reduce deteriorated charge transport material. It is difficult to maintain chargeability, charge transportability and latent image retainability required for the electrophotographic photoreceptor while reducing the charge transport material in the surface layer. The chargeability can be maintained if a charge transport material is included in a photosensitive layer formed under the surface layer. However, since the charge transportability and the latent image retainability are required in the surface layer, they cannot be maintained if the charge transport material in the surface layer is reduced. Therefore, methods of maintaining the charge transportability and the latent image retainability while reducing the charge transport material in the surface layer are urgently required.
Even when there is little electrostatic deterioration, it is essential to reduce abrasion due to the mechanical hazard loaded on the electrophotographic photoreceptor in image forming process and prevent the surface thereof from being contaminated when the electrophotographic photoreceptor is used for long periods. A paper powder and an external additive of the toner adhere to (stick in) the photoreceptor to cause the contamination. The contaminated surface is not correctly charged or irradiated, resulting in occasional production of abnormal images. A photoreceptor having low mechanical durability is abraded at its outermost surface and a new surface sequentially appears to prevent production of abnormal images, but it is difficult to have a long life.
Therefore, it is very important for an organic photoreceptor to prevent surface contamination to have a long life. The mechanical durability of the surface layer is effectively improved to reduce abrasion, which includes a risk of surface contamination.
As a method of improving the mechanical durability of the surface layer of the electrophotographic photoreceptor, Japanese published unexamined application No. JP-2002-139859-A discloses an electrophotographic photoreceptor formed of an electroconductive substrate, and a photosensitive layer and a protection layer including a filler overlying the substrate.
In addition, methods of increasing hardness of the surface layer to improve the mechanical durability thereof are disclosed in Japanese published unexamined applications Nos. JP-2001-125286-A and JP-2001-324857-A. Hardness of the protection layer of a photoreceptor is increased therein because the photoreceptor is damaged with a particulate magnetic material undesirably transferred and pressed to the photoreceptor at a transfer part and a cleaning part when a magnetic brush charger is used.
Improving the mechanical durability of the surface of the electrophotographic photoreceptor is effective for decreasing abrasion thereof, but not always effective for preventing surface contamination. Even when the mechanical durability of the electrophotographic photoreceptor is improved, an extraneous matter contaminating the photoreceptor accumulates when used for long periods.
Because of these reasons, a need exist for an electrophotographic photoreceptor having both charge transportability and latent image retainability even with a small amount of a charge transport material in the surface layer, and high durability of good electrical properties and continuously producing high-quality images even when used for long periods.